A steam turbine for the generation of electrical power comprises a casing enclosing a rotating shaft (also referred to as a rotor) and a plurality of radially extending rows of blades affixed to the shaft. Pressurized steam directed onto the blades causes blade and shaft rotation. The serial steam path typically includes a steam inlet, a plurality of steam pressure zones within the turbine and a steam outlet.
The shaft of a steam turbine for generating electrical power is rotatably coupled to a rotating shaft of an electric generator such that rotation of the turbine shaft imparts rotational energy to the generator shaft. The generator comprises first conductive windings disposed on the shaft and responsive to a source of electrical energy, and second conductive windings disposed in a stator surrounding the shaft. Rotation of the generator shaft and the windings disposed thereon induces electrical current in the second conductive windings according to known electromagnetic voltage induction principles.
Typically, the turbine is segregated into a plurality of pressure zones between successive stages of stationary and rotating blade rows. The purpose of such turbine blade geometries and configurations is to maximize the energy derived from the steam flow, thus increasing the efficiency of the electrical generating plant, i.e., the steam turbine operative in combination with the electric generator.
All regions where the steam turbine shaft penetrates the turbine casing must be sealed to prevent the escape of pressurized steam from the casing. Further, to improve turbine efficiency and minimize shaft vibratory motion, it is desirable to avoid steam leakage along the shaft between adjacent zones of differential pressure surrounding the stationary and rotating blade rows.
It is therefore known to attach circumferential labyrinth seals to the turbine casing surrounding the turbine shaft to minimize axial steam-path leakage while providing sufficient clearance between the shaft and the seals to allow unimpeded shaft rotation. Two types of labyrinth seals are known. A first type comprises sealing fins mounted directly to the turbine casing. A second type comprises fins mounted in arcuate spring-backed seal carrier segments, wherein a plurality of such segments are arranged to form a circular labyrinth seal ring surrounding the turbine shaft and mounted within the casing. Generally, between four and twenty seal segments are required to circumferentially surround the turbine shaft. The spring-backed mechanism urges the fins of each segment radially inwardly toward the shaft.
Both types of labyrinth seals are disposed at selected axial positions along the length of the turbine shaft to minimize steam leakage between regions of differential pressure. The teachings of the present invention relate primarily to the spring-backed seal segments due to the smaller seal clearances associated therewith, but the teachings can also be applied to the sealing fins mounted to the turbine casing.
Each labyrinth seal ring includes a plurality of substantially parallel spaced-apart annular teeth, also known as seal fins, extending radially inwardly from the seal carrier segments mounted to the turbine casing. The distal end of each seal fin is disposed proximate the rotating turbine shaft, leaving a small clearance therebetween. A minimal clearance between the seal fins and the turbine shaft minimizes axial seal leakage and thus the leakage steam flow between differential pressure regions. Similar seals are also utilized to prevent steam leakage from regions where the turbine shaft penetrates the casing.
The seal fins act as flow constrictions, such that multiple parallel seal fins act in concert to reduce the axial steam flow leakage between differential pressure zones to acceptable levels. It is known, however, that notwithstanding the use of the labyrinth seal rings, some steam continuously enters and exits the seal rings with a flow component directed generally axially along the shaft.
It is also known that a component of the steam flow enters and exits the labyrinth seal ring structure in a circumferential direction, typically referred to as “steam swirl.” It is generally accepted that the swirl results from two principal causes: (1) a circumferential steam flow component imparted by steam exiting the most adjacent upstream (i.e., in the direction of higher steam pressure) turbine stage; and (2) a circumferential flow component produced by a frictional effect of the rotating shaft. The latter component is in the direction of rotor rotation, unless the rotor shaft speed is less than the steam velocity leaving the upstream blade, and is referred to as a forward running swirl. The former component is always in the direction of rotor rotation
When the turbine rotor is centered within a seal ring, the local circumferential steam leakage flow velocities are substantially equivalent at all points around the rotor circumference. Thus there is no net steam force to urge the rotor from its axial center of rotation. On the contrary, if the rotor is off-center, an area of a seal chamber (i.e., a region bounded by two successive seal fins and the adjacent region of the turbine rotor) increases in one circumferential region of the rotor and decreases in a diametrically opposite region. The steam experiences a higher drag force in the region of decreased size than in the region of increased size. The differential drag forces induce a net pressure difference, pushing the rotor in the direction of rotation around the center of the seal. Thus the rotor “whirls” about its geometric center.
The rotor whirl responds primarily to the entering swirl velocity and the steam density. When the turbine load increases, the destabilizing forces created by the swirl also increase with increasing steam density, as does the amplitude of the rotor whirl. The rotor whirl increase is monotonic with increasing turbine load, and can eventually exceed acceptable turbine vibration amplitude limits, requiring the operator to reduce the turbine load. This condition is exhibited as a high vibration amplitude at the bearings, exceeding normal operating limits.
One prior art approach for limiting rotor instability by reducing rotor swirl is disclosed in U.S. Pat. No. 4,979,755 entitled “Flow Dams in Labyrinth Seals to Improve Rotor Stability”. FIG. 1 herein illustrates certain pertinent elements of a steam turbine including a rotating shaft or rotor 10 conventionally extending through regions of varying pressure within the turbine, from a region of higher fluid pressure to a region of lower fluid pressure, and including a flow dam according to the '755 patent. The shaft 10 in FIG. 1 represents a portion of the rotating shaft (the blades are not shown in FIG. 1) that extracts rotational energy from the pressurized steam directed to the blades.
A portion of two seal rings 12 (only two are illustrated for exemplary purposes in FIG. 1) are disposed axially along and circumferentially surrounding the shaft 10. The number of seal rings utilized in a turbine depends on various operational factors including the pressure to be sealed and the desired sealing efficiency.
Each seal ring includes a plurality of curved seal ring segments 14. In one embodiment, each of the seal ring segments subtends a 90° circumferential arc and thus a seal ring comprises four circumferentially adjacent seal ring segments 14. In other embodiments, the seal ring comprises more than four seal ring segments for surrounding the shaft 10. The seal rings 12 circumferentially surround the shaft 10 to minimize fluid leakage between regions of differential pressure through which the shaft 10 extends. For example, the seal rings 12 may form shaft end seals for a high-pressure end of a conventional steam turbine. Each seal segment 14 fits within a corresponding groove 16 formed in a stationary portion or casing 18 of the turbine.
Each seal segment 14 includes a biased backing member (not shown) to urge the seal segment 14 radially inwardly toward the shaft 10 by applying a force between mating surfaces 19A of the seal segment 14 and surface 19B of the stationary portion 18. Each seal segment 14 further comprises a shoulder 14A to limit inwardly directed travel of the seal segment 14.
A plurality of substantially parallel spaced-apart annular seal fins 20 are mounted on a radially inward face 14B of each seal segment 14. The annular seal fins 20, which are also referred to as seal legs, strips or teeth, surround the shaft 10 to provide a barrier against axial steam flow. The seal fins 20 are formed either as an integral element of the seal segment 14 or are retained by known peening, caulking or frictional techniques within slots formed in the seal segment 14.
The fins 20, typically constructed of stainless steel, are not intended to contact the shaft 10, but extend radially inward to within a relatively close proximity thereof to maintain a small working clearance between the shaft 10 and the fins 20. In one embodiment, this clearance is about 0.030 inches. An annular chamber or cavity 22 is defined between two successive fins 20.
In another embodiment the fins 20 can be mounted opposite raised lands (not shown) on the rotating shaft 10 to provide the axial sealing.
As described above, steam flowing circumferentially with respect to the shaft 10 within the cavities 22 can have a destabilizing effect on the shaft or rotor, creating rotor whirl when the steam flow is in the same direction as rotor rotation and when an eccentricity is present in the seal radial clearance.
To reduce steam swirl flow that can lead to the destabilizing rotor whirl, each seal segment 14 further comprises a flow dam 26 affixed to an end surface of a seal segment 14. Each seal segment 14 may further comprise a plurality of threaded bores for engagement with correspondingly threaded fasteners, such as flat-head machine screws 30 as shown in FIG. 1 to affix the flow dam 26 to an end surface. Each of the flow dams 26 is mounted perpendicularly to the seal fins 20 and attached to the seal segment 14 by insertion of the screws 30 into the threaded bores. The flow dams 26 substantially reduce the circumferential fluid flow in the cavities 22, thereby reducing the steam swirl condition.
In this prior art technique for limiting steam swirl and thus rotor whirl, the number of flow dams 26 is limited to the number of seal segments 14 comprising a circumferential seal ring 12, since each seal segment 14 accommodates one flow dam 26. Thus for example in the embodiment where four circumferentially adjacent seal segments 14 comprise a seal ring 12, only four flow dams 26 can be accommodated. This limitation may not, in some applications, sufficiently reduce the steam swirl, as the swirl reduction is directly dependent on the number of flow dams disposed around the shaft circumference. Swirl reduction also depends on the degree to which each flow dam closes off the cavity 22, i.e., the degree to which the flow dam reduces the gap between the shaft 10 and a radially inwardly facing edge 26A of the flow dam 26.